Fix equality delete load notification deadlock

[fallintoplace]

Summary

• Reuse an existing equality-delete load notifier instead of overwriting it with a new one.
• Keep the loader path scoped to the existing start-load state machine.
• Add a regression test that polls a waiter before the load is inserted, then verifies it is notified when the predicate is loaded.

Root cause

try_start_eq_del_load marked a delete file as Loading with one Notify, but insert_equality_delete replaced the map entry with a different Notify. Any reader that observed the first notifier could wait forever because only the second notifier was signaled.

Tests

• cargo fmt --check
• CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse cargo test -p iceberg test_equality_delete_waiter_is_notified_after_load_started --locked
• CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse cargo test -p iceberg arrow::delete_filter::tests --locked
• CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse cargo test -p iceberg arrow::caching_delete_file_loader::tests --locked

[viirya]

Nice catch — this is a real deadlock. try_start_eq_del_load creates notifier A, stores Loading(A), and returns A; the caller then calls insert_equality_delete, which (on main) creates a second notifier B and overwrites the entry with Loading(B). A reader that called get_equality_delete_predicate_for_delete_file_path in the window between those two steps captures A and awaits A.notified(), but the load only ever calls B.notify_waiters() — so that reader waits forever.

The fix has insert_equality_delete reuse the existing Loading notifier instead of minting a new one, which closes the window. Reusing the start-load notifier (rather than, say, having try_start_eq_del_load return it for the caller to thread through) keeps the call site simple and is the minimal correct change. The _ => { mint a new notify } fallback preserves behavior for callers that reach insert_equality_delete without a prior Loading entry.

The regression test is well-targeted: it polls a waiter to Pending before the load is inserted, then asserts the waiter is notified — exactly the race that was broken. LGTM.